"""author: wenyao
   data: 2020/10/20
   project: ludingji
"""

import math
a = int(input("请输入边长a:"))
b = int(input("请输入边长b:"))
c = int(input("请输入边长c:"))

#计算半边长
p = (a + b + c) / 2
#计算面积
s = math.sqrt(p*(p-a)*(p-b)*(p-c))
print(s)